As measured by their frequency, influence on quality of life and economic cost, hip fractures are a public health problem of crisis proportions. The exponential increase with age in hip fracture incidence, coupled with demonstrated age-dependent reductions in bone density and strength have led to the widely held view that age-related bone loss, or osteoporosis, is the most important determinant of hip fracture incidence. However, some have suggested that the increased propensity for falls among the elderly is the most important etiologic factor. Recently, through falls surveillance studies and in-vitro strength tests of cadaveric femora, we have shown that the energy available in a fall from standing height is about sixteen times the energy required to fracture the hip in-vitro. Moreover, most fallers who fracture their hip fall to the side, land directly on the hip and do not use the outstretched hand to break the fall. These findings suggest that in addition to the increased incidence of falling, the mechanics of the fall itself may well dominate the occurrence of hip fracture in the elderly. While previous research has determined those host and environmental factors which initiate falls, no previous work has focused on the role of fall mechanics in the etiology of hip fracture among the elderly. We hypothesize that reflex-mediated falls (in which neuromuscular response mechanisms are active) result in fall configurations and impact forces which represent a significantly decreased risk for hip fracture when compared to limp falls (in which neuromuscular response mechanisms are absent). To explore this hypothesis we will use a rapidly displaced trampoline to initiate safe falls in young adult volunteers. Kinematics of reflex-mediated and limp falls will be monitored using high-speed video in: a) young adult volunteers; b) an instrumented automotive crash dummy; and c) elderly cadaveric specimens. Our second hypothesis is that hip impact forces resulting from a fall can be predicted from a simple non-injurious experiment. This "pelvis-release" experiment allows us to characterize the combined influence of body segment configuration (the effective mass) and soft tissues overlying the hip by determining the spring constants and damping factors for simple mass-spring-dashpot models of the system. This experiment will be conducted with male and female volunteers representing a range of ages and body types for several potential impact configurations and both with and without muscle activity. The predictive accuracy of the model at realistic values of fall impact velocity will then be evaluated using cadaver drop tests. Finally, to provide a theoretical framework for understanding and extending the results of both the experimental falls and the pelvis-release experiments, we will develop a series of analytical models for falling and impact. These will begin with simple lumped mass representations and extend to two- and three-element articulated segments. We will also adapt a 15-segment, dynamic model (developed originally for automotive crash simulations) to the study of falling. Model predictions will be validated by comparison to the experimental falls and to cadaver impacts, and then used to explore conditions such as reaction times and decreased lower extremity strength which lead to high-risk falls in the elderly. We expect these experimental and analytical studies of fall mechanics to lead to an improved understanding of those factors which lead to falls with a high risk of hip fracture and thus to the design of more effective intervention strategies to reduce the growing incidence of hip fractures among the elderly.